The Vertical Distribution of Atmospheric BrO from Ground-Based Measurements

Robyn.Schofield [ at ]


Ground-based UV-Visible measurements targeting BrO were made at Lauder, New Zealand(45.0±S, 169.7±E) and Arrival Heights, Antarctica (77.8±S, 166.7±E). Differential OpticalAbsorption Spectroscopy (DOAS) was used to determine differential slant column densities(DSCDs) from the radiance measurements. UV-Visible measurements have beenmade in the two complementary viewing geometries of direct-sun and zenith-sky.A spherical curved earth single scattering radiative transfer model was developed. Theeffects of refraction, molecular absorption, Rayleigh and Mie scattering were included.Singularity at the tangent point was avoided and a complete intensity calculation performed.The DSCDs for both the direct-sun and zenith-sky viewing measurements werecalculated with this forward model.A general optimal estimation retrieval algorithm was developed to retrieve altitudeinformation by combining DSCDs from the direct-sun and zenith-sky viewing geometries.A complete retrieval characterisation and error analysis was performed. The characterisationillustrated that tropospheric sensitivity was obtained from the direct-sun viewingmeasurements, while the zenith-sky measurements were essential for stratospheric sensitivity.Stratospheric and tropospheric BrO columns were retrieved for the diurnal stages of80±, 84± and 87± SZA for Lauder. The diurnal and seasonal variation of the stratosphericcolumn was successfully retrieved from the measurements. The stratospheric columnswere consistent with a stratospheric Bry loading of 20 ppt. The tropospheric BrO columnretrieved over Lauder was less than 0.9 ppt if a uniform distribution throughout the troposphereis assumed. This is consistent with, though lower than, previous estimates ofBrO in the free troposphere of 0.5-2.0 ppt (Richter et al., 2002).The results of a ten week measurement campaign at Arrival Heights for the spring2002 are presented. Stratospheric and tropospheric BrO columns were retrieved at 80±,84± and 88± SZA. A high variability was observed for the retrieved stratospheric columns,due in part to the unusual stratospheric warming in the Antarctic spring 2002 (Allenet al., 2003). A mean ubiquitous tropospheric background of 0.3 ppt was retrieved. Alsoa bromine explosion event was observed, corresponding to a BrO mixing ratio of 7 pptfor a uniformly mixed boundary layer.

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Schofield, R. (2003): The Vertical Distribution of Atmospheric BrO from Ground-Based Measurements , PhD thesis, University of Auckland, New Zealand.

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